Changing Email forward address produces HTTP 500 error

Hello!
A client just reported me this bug: if you edit a mail user and want to change the forwarding address, you will get an http 500 server error. If you load the mail user page again, the new address shows there, however mail gets still forwarded to the old one. The workaround seems to be to first deactivate forwarding, then reactivate it with the new address, that way it will work.